1. Field of the Invention
The present invention relates to an optical recording medium having a plurality of recording layers.
2. Description of the Related Art
Optical recording media such as compact discs (CDs) and digital versatile discs (DVDs) are widely utilized as information recording media. In recent years, attention has also been given to optical recording media that utilize blue or blue violet laser light as the irradiation light so that a larger amount of information can be recorded than that available using convention laser light systems.
Optical recording media are generally classified into the following types: a ROM (Read Only Memory) type in which data cannot be recorded or rewritten, an RW (Rewritable) type in which data can be rewritten, and an R (Recordable) type in which data can be recorded only once.
In the RW and R type optical recording media, data is recorded by irradiating a recording layer with laser light to form recording marks that have a reflectivity different from that of surrounding space areas. Data is reproduced from a typical optical recording medium by irradiating a recording layer with laser light and detecting the difference in reflectivity between the recording marks and the space areas with a photodetector.
Such optical recording media can be provided with a plurality of recording layers, therefore resulting in higher recording capacities.
In the case of a multilayer optical recording medium having a plurality of recording layers, irradiation light must be transmitted, at least in part, through all the recording layer(s) other than the one farthest away from the light incident surface. The inventors have prototyped multilayer optical recording media having n layers (where n is 3 or 5) and found the following. When recording information on the recording layer (the L0 layer) farthest away from the light incident surface, for example, the light that is transmitted through the Ln-1 to L1 layers to reach the L0 layer undergoes a change in amount far greater than conventionally expected, depending on whether or not recording marks are already formed on the recording layers that the laser light is to be transmitted through, i.e. those that lie closer to the light incident surface (being the L1 to Ln-1 layers).
It is easily conceivable that the greater the number of layers, the greater the change in transmittance that will occur between when recording and when not recording, in proportion to the number of recording layers required to transmit light. If the number of recording layers increases from two to three, or to five for example, the number of recording layers necessary to transmit light also increases from one to two, or to four. The inventors have found that the rate of change in transmittance before and after recording may increase to as much as two or four fold. This has revealed the problem that the recording conditions of the respective recording layers vary to a degree that is greater than expected, depending on the presence or absence of recording marks on the L1 to Ln-1 layers, and that satisfactory recording cannot be necessarily undertaken, even if under identical recording conditions.
It has also been found that this propensity may even become more pronounced, i.e., the rate of change in transmittance to the L0 layer varies significantly in recording media having three or more layers, particularly when the L1 layer is already recorded.